Rock cavity

ABSTRACT

A rock cavity for storing fluids, solid products for some other purpose, such as the protected manufacture or production of goods, comprising a substantially vertical, cylindrical rock cavity comprising a conical top section (14), and a conical or horizontal bottom section (15) and a vertical section (1) extending therebetween and presenting in cross-section a polygonal shape, wherewith vertical shafts (11) are located in at least half the corners of the polygon.

TECHNICAL FIELD

The present invention relates to a method for constructing cavities inrock formations, and particularly, although not exclusively, verticalcylindrical cavities intended for storing in rock petroleum products, orother fluids, or solid products, such as chemicals, chemical waste,radioactive waste, and other materials which are suited for storage inrock cavities.

The object of the present invention is to provide such a method whichwill enable vertical cylindrical cavities to be constructed, blasted, inrock formations with the minimum explosive effect on residual cavitywalls, while observing maximum safety conditions with respect to theworking environment of the personnel involved in the construction ofthese cavities.

BACKGROUND PRIOR ART

It is previously known to store petroleum products, and also otherliquids lighter than water, in a cavity formed in groundwater carryingrock formations, in which the stored liquid lies in direct contact withthe water-permeable surface of the cavity walls. The pressure exerted onthe cavity walls by the liquid stored in the cavity is lighter than thepressure exerted by surrounding groundwater, thereby counteracting anytendency of the stored liquid to pass through the wall.

When the stored liquid is lighter than water and insoluble therein, itis a normal practice to provide a water bed in the lowermost region ofthe cavity.

SE-A-7802027-8 and 7901278-7 describe and illustrate complexes forstoring petroleum products and other fluids in rock formations. Thesestorage complexes, or locations, have a very high storage capacity,despite being of relatively small horizontal extension. The storedproduct is therewith located within a concentrated area, and theexpedient of shielding the storage area with a curtain of denselypacked, waterfilled drill holes can therefore be more readily carriedout, thereby to off-set lowering of the groundwater level and preventingthe stored product from spreading to the complex surroundings.

According to these patent specifications, the cavities are located atsubstantially mutually equal depths, and when seen in horizontal sectioneach cavity has a substantially circular or oval shape, and when seen inhorizontal cross-section throughout the whole of the complex the meanpoints of the circular or oval horizontal sections of respectivecavities lie in the corners of regular polygons, all having the samenumber of sides.

By regular polygon is meant a polygon in which all sides are of mutuallyequal length and all corner angles are the same. A regular polygon canthus be inscribed in a circle which passes through all of the cornerpoints and the centre of which thus also forms the centre of thepolygon.

In accordance with one embodiment of the invention these polygons havethe form of various sized pentagons having a common centre point. Thecavities are therefore arranged in concentric circles. A further cavitycan be arranged so that its centre axis coincides with the centre pointof these circles.

It is also known from SE-A-8300185-9 to construct in rock formations afluid-storage cavity location in which the actual cavity has the form ofa substantially vertical cylinder, around which there is provided aseries of vertical holes forming a water-drainage shield; this drainageshield is intended for removal of the water bed upon which the storedfluid has rested.

Present day rock cavities for storing oil or petroleum products have theform of long "loaves", i.e. horizontally extending rock cavitiespresenting a bottom surface area of 500×25 m or thereabove, and a heightof 30 m. It has been found that when storing oil products in rockcavities of this kind, in which the oil rests on a bed of water,microorganisms grow in the boundary layer between water and oil, theoil/oil products being destroyed thereby and rendered worthless forfuture use. When such cavities are used to store refined products, ithas been found necessary to re-refine the products in order to guaranteetheir usefulness.

In order to overcome these problems associated with horizontal cavities,the use of vertical, substantially cylindrical rock cavities has beenproposed, as beforementioned. Examples of such vertical cavities aredescribed and illustrated in SE-A-7901278-7 and SE-A-8300185-9, and insubsequent articles by K. I. Sagefors et al, WP-System, Stockholm,Sweden. When excavating the rock in the construction of such verticalcavities, there is first formed a top tunnel from which the conicallyshaped roof cupola is taken, by first drilling holes oliquely outwardsand downwards in the vicinity of the peripheral surface of the ultimatecupola or roof structure, filling these holes with explosives andblasting the rock; forming one or more transport tunnels so that saidtunnels open into the cylindrical peripheral surface of the ultimatevertical rock cavity, excavation of rock being effected from thetransport tunnels by vertical drilling and bench excavation operations,the shot rock-mass being taken-out at the bottom, which may taperconically downwards to a separate removal tunnel, which can be used forintroducing piping and like conduits into the cavity site, and forremoving goods stored in the cavity.

As mentioned above, previous methods for constructing substantiallycylindrical, vertical cavities from rock formations have involveddriving a top tunnel from which drilling takes place. This necessitatesthe provision of a large number of drill holes and therewith anexcessive charge of explosives, which places the roof of the cavityunder unnecessary strain. Construction of the top tunnel also results indisturbance of the rock located above the cavity, with subsequent riskof impaired strength.

Upon discovery that microorganisms grow in the boundary layer betweenthe product stored and the water present, it was demanded that any waterpresent should be kept to a minimum, and it was further proposed inconjunction herewith that the cavity walls be coated with an impreviouscomposite lining comprising multi-layers of shotcrete, reinforcedshotcrete, epoxy resin, glass fibre fabrics, and additionally epoxyresin. One such cavity-lining method is described by Beckers-Sigman,COLTURIET products.

It is not certain, however, that a lining of this nature would be ableto provide durable protection should the lining be subjected constantlyto water pressure on the rock side thereof. Consequently, in order toguarantee prolonged resistance of the lining, additional methods havebeen proposed for eliminating ambient water (SE-A-8300185-9).

It has been found that bench blasting has a highly deleterious affect onthe residual cavity wall, and hence it is necessary, at high costs, tobolt the cavity wall and to line the same in order to achieve a durableresult. Bench blasting also results in the formation of microfissuresthrough which water in surrounding rock can enter the cavity.

Bench blasting also presents a serious risk to the working environmentof those responsible for drilling the holes.

For reasons of a technical and environmental nature, there has now beenraised a demand for a new method of excavating vertical rock cavities.

DISCLOSURE OF THE PRESENT INVENTION

It has been found that the present invention surprisingly meets all ofthese demands. The method according to the invention is characterized byconstructing from a transport tunnel an upper circumferential tunnel oflarger external diameter than the diameter of the substantiallycylindrical part of the ultimate rock cavity, at a level above thehighest ceiling level of said ultimate cavity; forming from a secondtransport tunnel a lower circumferential chamber of larger externaldiameter than the diameter of the substantially cylindrical part of theultimate rock cavity at a level which lies not higher than the ultimatelevel of the lowermost point of the ultimate rock cavity; connectingthese circumferential chambers by excavating a central vertical shaftand by excavating at least three vertical shafts at the periphery of theultimate cavity; ring drilling horizontally from the central shaft intothe central rock mass in the ultimate cavity; fan drilling horizontalholes in the outer rock mass in the ultimate cavity, from the verticalperipheral shafts, so as to form a polygon of drill holes in ahorizontal section through the ultimate rock cavity; drilling angledholes from the said peripheral shafts in a manner to form a conical roofarch and/or a conical base profile; and by blasting from the bottomupwards in a consecutive series of blasting operations, to form apolygonal, vertical rock cavity.

These and other characteristic features of the invention are set forthin the following claims.

When excavating cavities in rock formations in accordance with theinvention, serious crack formations are less likely to occur in thecavity walls. In addition, all drilling work is effected from thevertical shafts; the drillers are located in the protection of theshafts and therefore need never enter the cavity. Blasting can beeffected from below, wherewith the drillers can quickly re-take theirworking position in the shafts.

The invention will now be described in more detail with reference to theaccompanying drawings, in which

FIG. 1 illustrates in horizontal section a preferred embodiment of arock cavity constructed in accordance with the invention;

FIG. 2 is a vertical sectional view of the embodiment illustrated inFIG. 1;

FIG. 3 is a horizontal sectional view of the upper part of the rockfacility according to FIG. 1;

FIG. 4 is a horizontal sectional view of a complex comprising aplurality of rock cavities according to FIGS. 1-3;

FIGS. 5-11 illustrate various sequences in constructing the cavity fromrock formations, wherein

FIG. 5 illustrates the drilling of holes from the central shaft;

FIG. 6 illustrates the drilling of finer holes in the outer part, andthe drilling of drainage holes from the peripheral shafts;

FIG. 7 illustrates the scaling of the cavity walls with water under highpressure;

FIG. 8 illustrates the step of spraying the cavity walls with shotcrete;

FIG. 9 illustrates the introduction of a platform into the centralshaft;

FIG. 10 illustrates the step of spraying the cavity walls with asynthetic resin composition; and

FIG. 11 illustrates inspection of the drainage system.

In the drawings the reference 1 identifies the periphery of an ultimate,substantially cylindrical and vertical cavity excavated from rock. Whenseen in horizontal section, the rock cavity has a polygonalcross-sectional form (in some cases a decagonal form). The ultimateouter contours of the cavity are shown in black, heavy lines, while thelighter drawn, full or broken lines show the cavity contours duringconstruction. A transport tunnel 2 opens into an annular chamber 3, thediameter of which, or at least its outer diameter, is greater than thediameter of the ultimate rock cavity (30-40 m), said chamber 3 beingconstructed from the transport tunnel 2. In the residual core mass 4located within the annular chamber or tunnel 3 there is now formed atunnel 5 which extends to a vertical shaft 6 intended for use as awaiting adit for horizontal/slightly sloping ring drilled holes in therock mass to be blasted in the excavation of the rock cavity. Whenconstructing the transport tunnel 2 there is formed at the same time asecond transport tunnel 7 which extends to the bottom level of theultimate rock cavity. A second annular tunnel 8 is excavated from thissecond transport tunnel 7 and the central vertical shaft 6 is joined tothe second annular tunnel 8 by means of a horizontal tunnel 9. Sidechambers 10 are excavated from the upper annular tunnel 3, inwardly ofthe rock mass. In the illustrated embodiments, three or six verticalshafts 11 are formed between the side chambers 10 and the lower annulartunnel 8, these shafts being formed by tunnel boring upwards from below.

This method involves drilling a narrow hole from above and downwards. Atunnel boring bit is connected in the tunnel 8 to a wire which extendsthrough the hole and which during drilling is drawn from the bottom ofthe hole upwards. When the drill bit has reached the top of the oncenarrow hole and the shaft has thus been completed, the drill bit islowered to the bottom of the shaft and moved to the site of the nextshaft, whereafter the procedure is repeated.

As beforementioned, holes 12 are ring drilled from the shaft 6horizontally into the rock mass to be blasted. In this case there isused a relatively coarse drill, diameter 10 cm, and drilling iscontinued to a distance of about 5 m from the ultimate cavity wall (40times the hole diameter in centimeters). Horizontal holes 13 are fandrilled from the shaft 11 into the rock mass which is to be blasted outand which has not been perforated from the centre. These holes aredrilled with finer drills, e.g. drills of 20-40 mm in diameter. Theoutermost drill holes 13a are instrumental in forming the inner wall ofthe ultimate rock cavity. These relatively fine holes are not normallydrilled to distances in excess of 10 m, since it is difficult to controlthe self-steering of the drill at distances greater than this.Consequently the sides of the polygon are seldom longer than 10 m.

Holes which are instrumental in shaping the ceiling or roof structure 14and the floor structure 15 of the cavity are drilled from the shaft 11.These holes are drilled from said shaft obliquely upwards and obliquelydownwards at an angle of from 45°-60°.

The holes are filled with blasting explosives upon completion of ahole-drilling sequence. The drill holes extending outwardly from thecentral vertical shaft are filled with heavy explosive charges, whereasthe holes drilled in the outer ring of rock-mass are charged with alighter querlite explosive charge, 11-17 mm in diameter.

Blasting is effected successively downwards, the rock is scaled and theshot rock-mass is taken out through the tunnel 9 with the aid of skipsor front loaders.

As soon as a blasting salvo has terminated, platforms can beautomatically lowered down the vertical shafts, whereafter the fallenrock debris can be sprayed with water from water cannons, to bind alldust. This significantly reduces the risk of silicosis.

In order to seal-off the rock-mass externally of the cavity complex,vertical holes 16 are drilled from the upper annular tunnel 3 straightdown through the rock-mass, to the level of the cavity bottom. A sealingagent is then injected into these holes, so as to fill the micro-cracksand macro-cracks in the rock-mass.

Subsequent to excavating the complete cavity, the rock-mass can bereadily sealed, by lowering lift or elevator platforms carryinghigh-pressure spray equipment down the peripheral shafts.

When desiring a more imprevious surface, the rock-mass can be treatedwith shotcrete from the same lift or elevator platforms as those used toscale the rock surfaces.

In certain cases, when storing fuel for civilian and military jetaircraft, totally impervious surfaces are required, so as to totallyeliminate the presence of water. In this case the cavity walls arecoated with a synthetic resin composite, from a collapsible/extendableplatform structure, which is lowered down from the mouth of the centralshaft and which comprises working platforms from which work can becarried out.

In order to eliminate the water pressure exerted by water in thesurrounding rock-mass, it is necessary to drain this water away. This iseffected by drilling drainage holes 17 in the rock-mass from theperipherally located vertical shafts 11. The drill holes 17 are placedso close together that water moving towards the rock cavity is capturedand carried away thereby. The holes 17 slope slightly downwards towardsthe vertical shafts 11 and discharge thereinto. The drainage water runsbehind a wall 18 cast in respective vertical shafts 11, and cantherewith be readily pumped away from the bottom of said shafts.Elevators can be mounted in the remaining part of the shafts 11, so thatthe shafts can be monitored with respect to water drainage.

Alternatively, when blasting is completed, the vertical shafts can befilled with a concrete construction, as illustrated in FIG. 1 by thereference 20. In this case the drainage pipes are led out through theconcrete construction. It will be understood that FIG. 1 onlyillustrates a few of the total number of drill holes required forblasting at each level.

The drainage holes 17 drilled behind the cavity walls 1 may suitably beconnected vertically at each corner of a polygon where no vertical shaftis located, by means of vertical holes 21. These vertical holes 21 mayalso be used to blow hot air through the drainage holes 17 in groups orsections, and in this way dry/heat the cavity wall prior to applying thesynthetic resin lining thereto.

In order to drain the ceiling region of the cavity, drainage holes 17are suitably drilled from the annular tunnel 3 at ceiling level, intowards the centre, as illustrated in FIG. 2. Conversely, for thepurpose of draining the bottom region of the cavity an umbrella ofdrainage holes 17 is drilled from the centrally located rock chute 22outwardly to an area externally of the cavity wall. Drainage water canbe removed from the rock chute 22 via pipes not shown.

The vertical shafts may comprise an active storage part of the overallstorage facility, or may alternatively play no part therein, dependingon the type of fluid to be stored. When storing jet fuel these shaftsplay no active storage part, and hence there is introduced into the rockchute above the drainage area a bottom structure through which pipes(not shown) are drawn for pumping away the jet fuel. When storing crudeoil, the whole of the tunnel and shaft system may form active storagelocations, in which case there is inserted in the tunnel 7 a plugthrough which oil-pumping pipes are drawn.

The complex illustrated in FIG. 4 thus includes a plurality of polygonalcavities formed in the rock-mass, each of these cavities having asubstantially cylindrical shape, and each cavity forming a storagespace, the rock-formed walls of which directly absorb the pressureexerted by the fluid stored in the cavity, the centre axes of thecavities extending vertically. Each cavity suitably has a verticalheight which is greater than or equal to its diameter in cross-section.

The storage complex is compact and requires the minimum of surface area.It is thus possible to construct very large storage complexes withinlimited areas. The area of the storage region is minimal. This greatlyfacilitates provision of those means required to avoid lowering of theambient ground water. The geometric configuration of the storage complexalso facilitates provision of water curtains externally of the storagecomplex. These water curtains comprise rows of vertical drill holesfilled with water. The groundwater level can be maintained within thestorage complex and externally thereof with the aid of such watercurtains. The fact that the storage complex can be constructed within acompact area enables the complex to be readily excavated from ahomogenous rock-mass, thereby avoiding disturbances in the surroundingsmore readily.

Since each cavity has a height which is greater than its diameter, therock-mass in which the complex is constructed can be utilized morefavorably to great depths, which enables a more compact storage complexto be constructed and a more favorable economy to be achieved withregard to the use of available ground area, and also provides improvedheat economy when the stored product is heated.

As a result of the considerable height of respective cavities, the headobtained is sufficient to enable the product to be readily pumped awaywith the aid of pumps arranged beneath said cavities. The compact designof the storage complex also means that the requisite pipe installationswill be less expensive than would otherwise be the case.

If the stored product is to be heated, this heat can be supplied to anydesired part of the cavities at any desired level.

If the stored products deposit slime or sludge, the sludge can readilybe collected and pumped away from the storage complex, and it is notnecessary to arrange large collecting spaces for the final deposition ofsludge in the bottom of the complex.

The particular form of the cavities also facilitates the installation ofmonitoring sensors, for example temperature responsive means and levelindicators, and the like. When the space is used as a machinery room,material transport can be effected with the aid of overhead cranes.

As beforementioned, the rock-mass can be sealed by injecting a sealingmaterial through drill holes. This sealing material may be a siliconeelastomer or the like.

Because the storage space is dry, it can also be used to store lowradioactive and average radioactive nuclear waste deriving from nuclearpower stations and nuclear research stations, in addition to theaforesaid products.

The rock cavity according to the present invention eliminates allproblems known at present within the oil storage technique. Thepumpability of oil stored compared with horizontal storage loavesprovides a volumetric gain in storage facilitates which can becalculated in multi-million sums of currency in storage costs over anoperational time of 20 years.

The method according to the invention affords the advantages of a rapidtunnel-driving method; precise contour drilling; optimal placement ofinjection holes; shot rock-mass can be removed independently ofdrilling; 80% of the drilled volume is coarse ring drilling; personnelneed not enter the rock cavity, because of the presence of the verticalshafts; worker protection and ergonometry is improved by the verticalshafts; shorter construction times in comparison with conventionaltechniques; and lower blasting costs. The costs saving compared withconventional technique for a rock storage complex having a volumetriccapacity of 500,000 m³ can be estimated to be at least 20 MSEK.

We claim:
 1. A method of constructing a substantially vertical,cylindrical rock cavity having a conical top part, a bottom part and avertical part having a cross-section in the shape of a polygoncomprising:(a) forming from a transport tunnel an upper circumferentialchamber having a radially inwardly extending chamber at a first level;(b) forming from a further transport tunnel a lower circumferentialchamber having a radially inwardly extending chamber at a second level;(c) forming a vertical central shaft to connect the upper and lowerradially inwardly extending chambers; (d) forming at least threevertical shafts; (e) ring drilling horizontally from the verticalcentral shaft; (f) drilling horizontally from the vertical shafts, saidhorizontal drill holes being located to form said cross-section in theshape of a polygon; (g) drilling angled holes from the vertical shaftsso as to form a conical roof or ceiling arch; and (h) blasting upwardsfrom below to form the vertical part of the rock cavity having thecross-section in the shape of a polygon wherein the upper and lowercircumferential chambers have outer diameters greater than thesubstantially cylindrical part of the rock cavity, the first level isabove the rock cavity, the second level is below the rock cavity andsaid at least three vertical shafts are formed at the periphery of therock cavity.
 2. The method of claim 1 further comprising formingdrainage holes external to the rock cavity and opening into the verticalperipheral shafts.
 3. The method of claim 1 further comprising drillingvertical holes downwardly from the upper circumferential chamber throughrock located exterior to the rock cavity and injecting the holes with awater-impervious substance.
 4. The method of claim 2 further comprisingconstructing a vertical wall in at least one of the vertical peripheralshafts to form a confined area into which the drainage holes discharge.5. The method of claim 1 further comprising drilling from the verticalperipheral shafts to form a conical bottom part of the rock cavity.